Matching two column values - mysql

Question

I have 2 tables. 'user_cities' and 'visits'. I want to check if a user visited a city which he was not supposed to visit.
CREATE TABLE `user_cities` (
`id` INT NOT NULL AUTO_INCREMENT,
`user_id` INT,
`name` varchar(255),
`city_id` INT,
PRIMARY KEY (`id`)
);
CREATE TABLE `visits` (
`id` INT NOT NULL AUTO_INCREMENT,
`user_id` INT,
`visit_id` INT,
`city_id` INT,
PRIMARY KEY (`id`)
);
INSERT INTO `user_cities` VALUES
(1, 1, 'John', 35),
(2, 1, 'John', 36),
(3, 1, 'John', 37),
(4, 2, 'Michael', 38),
(5, 2, 'Michael', 39);
INSERT INTO `visits` VALUES
(1, 1, 1, 35),
(2, 1, 2, 36),
(3, 1, 3, 38),
(4, 2, 4, 38),
(5, 2, 5, 39);
http://sqlfiddle.com/#!9/68c658
Example: John must visit only 35, 36 and 37. Michael must visit 38 and 39. These are defined in 'user_cities'
However, John has visited 38 (visits id 3)
How can i query users that visited the wrong city?

Answer 1

If you're not familiar with SQL this kind of problem is a bit trickier than it appears.
You want to select the information from the visits where that visit is not specified by user_cities. The usual way to do this is to use a LEFT JOIN to the user_cities table for the record that validates the visit, and then to find entries where the LEFT JOIN can't find a match.
So your query would be:
SELECT *
FROM visits
LEFT JOIN user_cities
ON(visits.user_id = user_cities.user_id AND
visits.city_id = user_cities.city_id)
WHERE user_cities.id IS NULL
Of course you would probably also extend the query to give you the name of the user (and the coty, but that's not in your example code)

Answer 2

You can try using left join
DEMO
select a.user_id,a.city_id
FROM visits a left join user_cities b on a.user_id=b.user_id and
a.city_id=b.city_id
where b.user_id is null
OUTPUT:
1 John 37

Answer 3

The only way I can see to do this is via a full outer join (which MySQL doesn't even support, so we have to use a workaround):
SELECT COALESCE(user_id_1, user_id_2) AS user_id_matching
FROM
(
SELECT
uc.user_id AS user_id_1,
uc.city_id AS city_id_1,
v.user_id AS user_id_2,
v.city_id AS city_id_2
FROM user_cities uc
LEFT JOIN visits v
ON uc.user_id = v.user_id AND uc.city_id = v.city_id
UNION ALL
SELECT
uc.user_id AS user_id_1,
uc.city_id AS city_id_1,
v.user_id AS user_id_2,
v.city_id AS city_id_2
FROM user_cities uc
RIGHT JOIN visits v
ON uc.user_id = v.user_id AND uc.city_id = v.city_id
WHERE uc.user_id IS NULL
) t
GROUP BY COALESCE(user_id_1, user_id_2)
HAVING
COUNT(CASE WHEN user_id_1 IS NULL THEN 1 END) = 0 AND
COUNT(CASE WHEN user_id_2 IS NULL THEN 1 END) = 0;
Demo